The invention relates to an internal combustion engine/turbocharger system.
An internal combustion engine/turbocharger system of this kind is known, for example, from German Patent document DE 4330487 C1. In this case, the guide vane array is arranged in the region in the form of an annular nozzle starting from the flow duct and opening onto the turbine rotor. The guide vane array comprises a variable guide vane array which is provided with adjustable guide vanes, allows free cross sections of flow of different sizes and can be used as a restrictor in a braking mode to determine the respective narrowest cross section of flow in the exhaust path to the turbine and to adjust it as a function of operating parameters of the internal combustion engine. With such a system it is possible to achieve high braking powers. However, there are problems in achieving these braking powers with low thermal loading on the internal combustion engine. It is therefore an aim of the invention to define a respectively suitable internal combustion engine/turbocharger system.
This aim is served by the invention, which is intended to provide internal combustion engine/turbocharger systems which will allow high braking powers for the braking mode with relatively low thermal loads on the internal combustion engine.
This is achieved in internal combustion engine/turbocharger systems by a design according to which the main parameters of the turbine are defined in such a way, in relation to the total displacement of the internal combustion engine, that maximum braking powers are made possible with comparatively low thermal loading of the internal combustion engine.
In this context, the following relation applies: ##EQU1##
where A.sub.T denotes the free cross section of flow exposed in the exhaust path to the turbine at maximum braking power, D.sub.T denotes the inlet diameter of the turbine rotor (in radial turbines, the geometric inlet diameter; in axial turbines, the mean diameter of flow of the design) and V.sub.H denotes the displacement of the internal combustion engine. TBF forms a turbo-braking factor, which, in the context of the invention, is less than 0.005 (5%) and is preferably 0.001 to 0.003 (1-3%).
Based on maximum braking power being the "design point", the free cross section of flow exposed in the exhaust path to the turbine at maximum braking power in the context of the invention is formed by (1) the cross section remaining as a residual cross section flow when the guide vane array is closed, covering a lower engine-speed range, and (2) a variable cross-sectional component, which is larger than that for the design point. The ratio of the additional, variable cross section of flow (which increases towards the maximum braking power) to the residual cross section of flow (remaining when the guide vane array is closed) is greater than 0.2 for the design point, namely maximum braking power, and preferably is in the range between 0.2 and 1.
With a small ratio, the braking power in the lower engine-speed range is relatively low and rises relatively steeply in the upper engine-speed range. With a large ratio, a higher braking power is obtained in the lower engine-speed range, with a less steep rise in the braking power towards the point of maximum braking power as the design point, in comparison with smaller ratios. Based on the engine-speed band, the upper speed range, in which an additional, variable cross section flow is exposed, begins at about 2/3 to 3/4 of the speed at maximum braking power. The speed at maximum braking power lies within a range which is about 1/4 to 1/3 times greater than the rated speed of the internal combustion engine, i.e. the speed at the maximum power of the internal combustion engine.